1. Field of the Invention
The present invention relates to a seismic survey method of the subsoil and, more specifically, a survey method of the subsoil which uses data obtained from seismic responses generated from various seismic sources.
2. Description of Related Art
In order to identify geological structures in the subsoil, in particular potential hydrocarbon traps, effecting so-called seismic acquisition campaigns is known, in which seismic waves are used for effecting measurements of the non-invasive type. In a seismic acquisition campaign, seismic waves are generated by means of artificial sources which can be obtained with explosives, vibrating mechanical devices, air cannons or with any other device suitable for generating seismic waves.
Seismic wave sources are positioned in multiple positions on the surface of the land or sea, above the area to be surveyed. These sources are activated to generate seismic waves which propagate in depth, moving away from the same sources, towards geological structures in the subsoil. During the propagation, the waves are also partially reflected towards the surface on the part of the structures to be surveyed. Once they have reached the surface, the reflected seismic waves are detected and transformed into electric signals by specific sensors (geophones or hydrophones) and then recorded. The sensors are situated on the surface in suitable configurations. The position of the sensors can also vary with a variation in the position of the seismic wave source.
For offshore seismic acquisition campaigns, so-called “seismic ship” is used. The ship tows the seismic source, immersed in water, and a series of cables, also immersed, along which the sensors are arranged. Possible variants to this scheme are obtained by using various ships contemporaneously, which entrain additional cables and/or sources. Different recordings are obtained by activating the seismic wave source in different positions along the navigation line and recording the data for all the sensors and for a finite time after each activation. The navigation line is suitably projected so as to guarantee coverage of the whole area to be surveyed. The combination of all the data recorded for the same position of the source forms the so-called seismic record or “shot gather”.
The signals registered during the seismic acquisition campaign are subsequently processed to obtain information relating to the structure and characteristics of the subsoil. The expression “seismic migration” refers in particular to the processing procedure which allows to obtain depth images of the separation surfaces between different rock layers starting from the seismic data. The migration is applied to the single shot gathers, producing a volumetric image of the subsoil for each shot gather. The shot gathers are therefore processed individually and the final image is obtained by summing the partial images produced by the migration of each single shot gather.
One of the disadvantages of this type of seismic survey is due to the fact that, in order to obtain an accurate survey of the subsoil through the migration of each single shot gather, numerous partial images must be summed. In this way, the cost of the seismic survey increases in proportion to the number of shot gathers to be migrated.
Furthermore, it should be pointed out that the calculation complexity due to the migration of each single shot gather is not negligible so that, in order to have reasonable operating times, considering the migration of a so-called “seismic dataset” of industrial dimensions (i.e. containing tens or hundreds of thousands of shot gathers), numerous supercomputers executing operations in parallel, must be used.
In order to make the seismic survey procedure more efficient, various shot gathers can be summed before their migration. In this way, however, the accuracy of the final image of the subsoil is jeopardized as the migration operation of the sum of the shot gathers is different from the summing operation of the migrations of each shot gather and, in addition, it introduces cross-talks into the final image itself.